1. Technical Field of the Invention
The present invention relates to improving the photostability of at least one dibenzoylmethane sunscreen compound with respect to UV radiation, by intimately admixing therewith an effective photostabilizing amount of a silane or organosiloxane compound bearing a benzylidenecamphor substituent.
2. Description of the Prior Art
It is known to this art that light radiation with wavelengths of from 280 nm to 400 nm promotes tanning of the human epidermis, and that irradiation with wavelengths more particularly ranging from 280 to 320 nm, i.e., UV-B irradiation, causes erythema and skin burns which can be harmful to the development of a natural tan. For these reasons, as well as for aesthetic reasons, a constant demand exists for controlling this natural tanning and, also, the coloration of the skin; such UV-B radiation should thus be screened from the skin.
It is also known to this art that UV-A irradiation, with wavelengths of from 320 to 400 nm, which tans the skin, is apt to induce an adverse change therein, in particular in the case of sensitive skin or skin which is continually exposed to solar radiation. UV-A radiation in particular causes a loss of elasticity of the skin and the appearance of wrinkles, promoting premature aging. Such irradiation promotes triggering of the erythemal reaction or enhances this reaction in certain individuals and may even be the source of phototoxic or photoallergic reactions. Thus, for aesthetic and cosmetic reasons such as conserving the skins' natural elasticity, for example, an ever-increasing number of subjects wish to control the effect of UV-A rays on their skin. It is thus desirable to also screen UV-A radiation from the skin.
In this regard, one particularly advantageous class of UV-A screening agents currently includes dibenzoylmethane derivatives, and in particular 4-(tert-butyl)-4'-methoxydibenzoylmethane, which have high intrinsic absorption power. These dibenzoylmethane derivatives, which are compounds that are well known per se as screening agents that are active in the UV-A range, are particularly described in FR-A-2,326,405 and FR-A-2,440,933, as well as in EP-A-0,114,607; 4-(tert-butyl)-4'-methoxydibenzoylmethane is moreover commercially available under the trademark "PARSOL 1789" from Givaudan.
Unfortunately, it has been determined that dibenzoylmethane derivatives are compounds that are relatively sensitive to ultraviolet radiation (especially UV-A radiation), namely, more specifically, they exhibit an annoying tendency to be degraded more or less rapidly under the influence of such radiation. Thus, this substantial lack of photochemical stability of dibenzoylmethane derivatives in the face of the ultraviolet radiation to which they are by nature intended to be subjected, does not guarantee constant protection during prolonged exposure to the sun, such that, in a restricting manner, repeated applications at regular and close time intervals must be carried out by the user in order to obtain effective protection of the skin against UV radiation.
Thus, EP-A-0,709,080 describes combining dibenzoylmethane derivatives with benzalmalonate derivatives in order to reduce the photoinstability of said dibenzoylmethane derivatives. Nonetheless, the photostabilization of dibenzoylmethane derivatives with respect to UV radiation constitutes, to date, a problem which has not yet been solved completely satisfactorily.
FR-2,607,996 and WO-94/04131 describe combining dibenzoylmethane derivatives with hydrocarbon-based benzylidenecamphor derivatives such as 3-(4-methylbenzylidene)camphor in order to reduce the photoinstability of said dibenzoylmethane derivatives.
Another difficulty, independent of the one indicated above, encountered with dibenzoylmethane derivatives is that these are lipophilic sunscreen agents with the particular feature, and also present the disadvantage of being solid at room temperature. Accordingly, formulating same into antisun cosmetic compositions entails certain constraints as regards their processing, in particular for determining solvents for properly dissolving same, whether alone or in combination with other screening agents. In this regard, oils have been typically employed such as esters, and more particularly C.sub.12 -C.sub.15 alkyl benzoates ("Finsolv TN" marketed by Finetex), or triglycerides, and in particular C.sub.8 -C.sub.12 fatty acid triglycerides ("Miglyol 812" marketed by Huls), but these various products exhibit solubilizing properties with respect to the aforesaid screening agents that remain insufficient.
Sunscreen formulations based on dibenzoylmethane derivatives and on 3-(4-methylbenzylidene)camphor as described in FR-2,607,996 and WO-94/04131 do not obviate this problem of solubility of said dibenzoylmethane derivatives completely satisfactorily.